# Does glass experience damage at a cumulative rate where slight damage 2 months ago can accumulate to damage/stress fractures or if I drop my phone does it depend on the direct and singular fall?

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I work as an smart phone repair tech, and was wondering, if I drop my phone 5x in a month do all the previous drops damage my phone to an unnoticeable level that eventually reaches a breaking point, or does only the acute fall dictate the damage (I know most phones have a robust glass, does this change the glasses historical tolerances?

In: Physics

Yes, virtually all materials experience fatigue damage; small growth of micro cracks whenever they’re loaded up. Brittle materials, like glass, tend to be better about this but it still happens.

What you are asking about here is a field of study known as “fracture mechanics” and your particular question fits into the “Griffith Flaw” theory.

Glass is very hard but very brittle. You may be creating microcracks when dropping your phone. A crack has a tip, which is the concentrated point, and surfaces of material created as the crack moves around the material, separating it and making more surfaces – this leads to an increase in stored energy in the material as the cracks puts some of the material into tension (a crack is two material faces being pulled apart). This energy is in some complex mathetmatical way related to the energy from when it was dropped. Think about pulling an elastic band apart, you are storing energy in the band and when you let go it snaps back quickly, if you pull the band too far it tears a little, you can still stretch it but not as much. Eventually you pull the band so far it tears in half and violently snaps around releasing the energy you put in.

For glass, and these microcracks to grow, you have to put more energy in, such as dropping the phone again. Eventually, a critical level will be reached where the crack is now so long that it can grow without much energy needing to be put into it. You could bump it against the table gently rather than drop it from a height for example. As these cracks are beyond a critical point, with a “Griffith Flaw”, it can propagate and release all the stored energy from the surfaces instantly, so when I said “grow without much energy needing to be put into it” I really meant it will smash as the material tries to reach a lower energy state, which unfortunately now is in pieces on the floor.

I hope that makes sense.

TL;DR: bit of both, cracks accumulate, grow, reach a critical level, and can result in a smash with only a tiny amount of additional energy which is tied to the growth energy requirements of the ‘critical’ crack.